The invention relates to the fields of electronics, integrated circuits, optoelectronics, and assemblies of functional blocks, and specifically to the self-assembly and mounting of these small functional blocks onto a substrate to form larger, compound electronic or opto-electronic assemblies.
There is a pressing need for new techniques of self-assembling small functional blocks onto a substrate. Two main factors drive this need. First, the economic of integrated circuit (IC) manufacturing is that of an economy of scale. The high cost of materials and processing must be divided among many, small functional blocks. Second, optimum device performance often requires that different functions of a device be preformed by different materials, each possessing unique properties. Unfortunately, different materials can require different processing, and these materials and processes are not often compatible. Hence elements that make up a device must be assembled after initial processing.
As an example of how an improved self assembly process can be beneficially applied to the first case, large area flat panel displays cannot be made cost effectively from single-crystal-silicon substrates. However, it is cost effective to construct large displays by mounting small, single-crystal-silicon transistors on low cost substrates. In this case the substrate must only provide electrical interconnection between transistors and mechanical support. As an example of the second case, optimized opto-electronic circuits may require the integration of silicon logic functional blocks with III-V semiconductor optical detectors or solid-state lasers. In this case, III-V semiconductor elements could function as functional blocks to be mounted in silicon based ICs functioning as substrate. Several methods and apparatuses for performing the mounting operation have been previously disclosed. All have specific limitations and shortcomings as discussed below.
A prior art approach is described by Yando in U.S. Pat. No. 3,439,416. Yando describes functional blocks or structures placed, trapped, or vibrated on an array of magnets. Such magnets include magnetized layers alternating with non-magnetized layers to form a laminated structure. Functional blocks are matched onto the array of magnets forming an assembly thereof. However, severe limitations exist on the shape, size, and distribution of the functional blocks. Functional block width must match the spacing of the magnetic layers and the distribution of functional blocks is constrained by the parallel geometry of lamination. In addition, self-alignment of functional blocks requires the presence of the laminated magnetic structure which puts severe limitations on the materials that can be used for both the substrate and functional blocks and can result in higher material cost. Furthermore, the structures disclosed by Yando typically possess millimeter sized dimensions and are therefore generally incompatible with micron sized integrated circuit structures. Accordingly, the method and structure disclosed by Yando is thereby too large and complicated to be effective for assembling a state-of-art microstructure of functional blocks onto a substrate.
Another approach involves mating physical features between a packaged surface mount device and substrate as described by Liebes, Jr. et al. in U.S. Pat. No. 5,034,802. The assembly process described requires a human or robotic arm to physically pick, align, and attach a centimeter sized packaged surface mount device onto a substrate. Such a process is limiting because of the need for a human or robotic arm. The human or robotic arm assembles each packaged device onto a substrate in a serial fashion, one device at a time and not simultaneously, thereby limiting the rate, efficiency, and effectiveness of the operation. Moreover, the method uses centimeter sized devices (or packaged surface mount integrated circuits), and would have little applicability with micron sized integrated circuits in die form.
Another approach, such as the one described in U.S. Pat. No. 4,542,397, Biegelsen et al., involves a method of placing parallelogram shaped structures onto a substrate by mechanical vibration. Alternately, the method may employ pulsating air through apertures in the support space (or substrate). Limitations to the method include an apparatus capable of vibrating the structures, or an apparatus for pulsating air through the apertures. Moreover, the method described relies upon centimeter-sized die and would have little applicability with state-of-art micron sized structures.
A further approach such as that described in U.S. Pat. No. 4,194,668 by Akyurek discloses an apparatus for aligning and soldering electrode pedestals onto solderable ohmic anode contacts. The anode contacts are portions of individual semiconductor chips located on a wafer. Assembling the structures requires techniques of sprinkling pedestals onto a mask and then electromagnetic shaking such pedestals for alignment. The method becomes limiting because of the need for a shaking apparatus for the electromagnetic shaking step. In addition, the method requires a feed surface gently sloping to the mask for transferring electronic pedestals onto the mask. Moreover, the method is solely in context to electrode pedestals and silicon wafers, thereby limiting the use of such method to those structures.
Another approach, that combines many aspects of the previous two approaches, is that of Smith et al. presented in U.S. Pat. Nos. 5,904,545, 5,545,291, 5,824,186, and 5,783,856. These patents disclose a method and apparatus for the self-assembly of functional blocks into pre-formed recesses in a substrate. Recesses are formed in a substrate prior to the functional block self-assembly process. The self-assembly process includes mixing functional blocks with a liquid to form a slurry, and then flowing this slurry over a prepared substrate. Functional blocks randomly move in the slurry and can fall into the recesses. The forces holding functional blocks into the recesses are weak, and no means of modulating these forces are discussed or evident. Significantly, no technique of applying additional forces are provided.
A prior art approach is described by Cohn in U.S. Pat. No. 5,355,577. Cohn describes a method and apparatus for the assembly of micro-fabricated devices which employs electrostatic forces to trap the devices and vibration to randomly move the devices over a substrate to the trap sites. The electrostatic forces are generated by high voltage biased electrodes that substantially cover the substrate and are arranged is a parallel-plate capacitor. Devices are attracted and trapped at apertures formed in the upper electrode.
FIG. 1 illustrates a general arrangement of the method disclosed in the U.S. Pat. No. 5,355,577. This figure illustrates a high voltage supply 50 powering a parallel-plate capacitor electrode arrangement with electrodes 66 and 70 substantially covering both sides of a substrate 68. Apertures 64 and 65 trap functional blocks 56 due to electrostatic field lines 72.
Limitations to this patent include a use of a parallel-plate electrode geometry and electrodes that substantially cover the substrate. The resulting geometry has several consequences. First, it requires at least three layers of materials to prepare a substrate for device tapping, a bottom conductor, an insulating layer, and a top conductor. Second, it is unlikely that the extensive electrodes used for trapping will be compatible with most finished products, hence additional process steps must be added to remove these electrodes after trapping. Further, once the tapping electrodes are removed, the trapping process cannot be reworked, repeated, without reforming the electrodes. And if the possibility of rework is to be allowed, then the trapping electrodes cannot also be used as electrical interconnects between devices, as the high trapping voltage recommended xcx9c8 kV, is not compatible with microelectronic functional blocks. An additional limitation resulting from use of parallel-plate electrodes, which substantially cover the substrate, is that there is no easy, electrical means of determining when all trapping sites are occupied. This is because, in the typical applications envisioned, the change in electrode capacitance with each trapped particle is very small relative to the total capacitance of the electrodes. Hence the change in electrical characteristics is too small to detect. A further limitation resulting from use of electrodes which substantially cover a substrate and use of the suggested high trapping voltage is that a majority of the substrate is subjected to very high electric field strength, greatly increasing the odds of failure due to dielectric breakdown. Hence the substrate material and construction must be of a universally high quality, with resulting higher costs.
It must also be noted that devices will not align over apertures as claimed if the thickness of the substrate is less than xcx9c10 microns. Further, the high bias voltage presented is not compatible with materials of this thickness. Hence the electrode geometry claimed in the U.S. Pat. No. 5,355,577 cannot be construed to include the miniature and planar-electrode. The reason alignment fails is that, during mounting, devices are not ohmically connected to the top electrode, but rather are capacitively coupled. Cohn does not discuss the type of electrical connection in the U.S. Pat. No. 5,355,577, but given the small size of devices envisioned, and the weak forces employed, achieving an ohmic connection is not tenable.
In the U.S. Pat. No. 5,355,577, Cohn also discloses an alternate embodiment using a multitude of planar electrodes to form a negative dielectrophoresis trap. The method attracts functional blocks to regions of relatively lower electric field strength. However, a limitation exists during mounting in that functional blocks must have a lower average dielectric constant than that of the surrounding medium. Furthermore, the required electrode configuration is complex and must cover the entire substrate surface. Hence all the limitations discussed above regarding xe2x80x9celectrodes that substantially cover the substratexe2x80x9d still apply here, plus limitations regarding the additional complexity of the electrode configuration.
Finally, there are many previous art disclosures that employ electrostatic chucks for various uses. Most are employed to hold a silicon wafer during IC processing, see for example, U.S. Pat. Nos. 4,184,188, 4,724,510, 4,520,421, 5,539,179, and 4,962,441, and are not amendable to mounting sub-millimeter size functional blocks. Another use proposed for electrostatic chucks is to deposit particles for chemical and pharmaceutical manufacture, U.S. Pat. No. 5,858,099. While they do employ electrostatic chucks, these disclosures do not apply to the present application.
Even with the techniques above, the concentration of functional blocks upon regions of receptor sites is not good enough. Several passes of the web through the self-assembly process is required to completely fill all of the receptor sites. In the U.S. Pat. No. 5,355,577, the functional blocks only move about in a random fashion over the substrate. Thus, it is difficult to control the incident of the functional blocks upon the receptor sites.
There remains needs for rapid and efficient mounting of one or more small functional blocks on a, potentially large, substrate, while using materials, processes, and structures that are easily and economically integrated with existing flat panel display and microelectronic manufacturing processes.
Methods and apparatuses employing electric and magnetic forces and to attract, guide, align and securely clamp functional blocks at pre-determined locations on a substrate are disclosed.
In one exemplary embodiment, a substrate is provided with receptor sites wherein each of the receptor sites is designed to couple to one of the functional blocks. Electrodes are coupled to the substrate. The electrodes cover the receptor sites such that portions of the receptor sites are coated with the electrodes. A voltage source is then applied to the electrodes using a first electrical circuit such that each electrode has a voltage different from that of another electrode. The electrodes form an electric field. And, the functional blocks having electronic devices in a slurry solution are dispensed over the substrate wherein each block is fabricated out of a material having a high dielectric constant such that said functional blocks are attracted to the higher field strength regions of the electric field and are guided to the receptor sites.
In another exemplary embodiment, a substrate is provided with receptor sites wherein each of the receptor sites is designed to couple to one of the functional blocks. Multiple strips of electrodes are coupled to the substrate such that there is a slot separating the strips of electrodes from each other. A voltage source is applied to the strips of electrodes using an electrical circuit such that an electric field is generated in the slot. The functional blocks are designed such that each of the functional blocks has low dielectric constant regions that cause the functional blocks to be repelled from the higher field strength regions of the electric field and hence pushed away from the slots. The functional blocks, in a slurry solution, are then dispensed over the substrate wherein each of the functional blocks includes an electronic device and wherein the three strips of electrodes guide the functional blocks to the receptor sites.
In yet, another exemplary embodiment, a substrate is provided with receptor sites located on a first surface of the substrate wherein each of the receptor sites is designed to couple to one of the functional blocks. A first electrode plate is coupled to the first surface and the first electrode plate does not completely cover each of the receptor sites. A second electrode plate is coupled to a second surface of the substrate. The first electrode plate connects to the second electrode plate. A voltage source is applied to the first electrode plate and the second electrode plate such that an electric field is generated in each of the receptor sites in a region not covered by the first electrode plate. The functional blocks are made out of a high dielectric constant material and has a top surface, a bottom surface, and side surfaces. The functional blocks, in a slurry solution having an intermediate dielectric constant, are dispensed over the substrate. The top surface and the side surfaces of the functional blocks are coated with a low dielectric constant material such that the top surface and the side surfaces are repelled from higher field strength regions of the electric field and the bottom surface is attracted to the higher field strength regions of the electric field facilitating proper couplings of the functional blocks to the receptor sites.
In yet, another embodiment, a substrate is provided with receptor sites on a first surface of the substrate wherein each of the receptor sites is designed to couple to one of the functional blocks. The substrate is positioned relative to a pair of high magnetic permeability strips fabricated out of materials with a high magnetic permeability, such that the pair of high magnetic permeability strips not being permanently coupled to the second surface. A magnetic field is applied to a volume of space including the pair of high magnetic permeability strips such that a region of high magnetic field gradient is produced between each member of the pair of high magnetic permeability. The functional blocks are then dispensed in a slurry solution over the first surface. Each of the functional blocks, being made out of a material having a low magnetic permeability property, includes an electronic device, a top surface, a bottom surface, and side surfaces wherein the bottom surface is coupled to a high magnetic permeability layer, and, the bottom surface is attracted to the region of high magnetic field gradient facilitating proper coupling of the functional blocks to the receptor sites.
And, in another exemplary embodiment, a method to clean excess functional blocks from a substrate includes positioning a bottom surface of the substrate over an electrode layer. The electrode layer being fabricated of patterned conducting material and supporting electrically insulating material. The substrate includes the functional blocks properly deposited and bound to receptor sites located on a top surface of the substrate. Excess functional blocks are on the top surface. A voltage source is applied to the electrode layer such that an electric field is formed clamping the functional blocks that are already bounded to the receptors sites in these receptor sites. Lastly, the excess functional blocks are swept off the top surface.